What size wiring for Central A/C compressor?

[Mikepier]

I'm running new electric in a friends basement and so far uncovered a lot of bad wiring jobs, buried splice boxes, you name it. So now I noticed that the central A/C compressor is wired with 10 guage wire. I'm pretty sure its suppose to be 8 guage. And also the existing 10 guage wire is hooked up to a 40 A breaker, which of course is not right, its suppose to be a 30A breaker. So since everything is wide open now, I was wondering should I run new 8 guage wire to the A/C? The unit is a Lennox, I beleive a 3 ton unit, and the plate says "Min circuit ampacity 24.4 amps" and it also says " Max fuse or ckt bkr 40A". Even if the unit has been running fine all these years, I'm wondering what happens down the road if it needs to be replaced with a new unit that needs more amperage. the existing 10 guage wire will not be sufficient I'm thinking.

[gfretwell]

OK hold on to your hat.

That label says max fuse or breaker 40a ... that is what you have

It also says minimum circuit ampacity 24.4a so the wire can be 12 ga.

You are larger than you have to be with 10ga.

That is what happens in article 440. They know you need a big breaker to get the compressor going but the actually running current is much lower. If you look at 310.15 you will see #12 is actually rated at

25a,. The common axiom that 14ga = 15a, 12ga = 20a comes from 240(4)(D) where they build the 80% safety factor in. (it is actually the max breaker size) It does not apply on dedicated motor circuits Homeowners will keep plugging things in until the breaker trips, then unplug the clock.

That label has it built in already by the engineer. You will see that compressor actually runs more like 16-18a on a hot day.

[bob haller]

normally a new install like a replacement AC will get all new wring and service disvconnect. a new 8 gauge copper line with new disconnect by the compressor will see less voltage drop, a good thing.

but new compressors will likely be more efficent and run with less current.

if everything is open then this is the best time to upgrade, and if breaker space is available run a couple 20 amp lines to work boxes for future use if needed. I did that once and a few years later was glad I did:) when we decided to install a new kitchen

[The Daring Dufas]

In The U.S., standard gauge copper wire for a 20 amp circuit is 12 gauge and 10 gauge for a 30 amp circuit. Below is a link to a chart for standard current ratings for home wiring. There are all sorts of different ratings for different insulation types, wire in free air, in conduit or buried but the chart shows what any electrician would go by if wiring a home. Aluminum wiring is usually one gauge size larger for the same current carrying capacity as copper. ^_^

TDD

[Mikepier]

I understand what the article is saying, but at the very least, shouldn't the 10 guage wire be on a 30A breaker and not 40A?

[Dean Hoffman]

The rules are different for motors than on other things for what it's worth. Wire is supposed to be sized at 125% of the full load amperage shown on the appropriate chart. Overcurrent protection can be

175% of the value shown in the chart. One can go to the next larger size if the 175% calculation puts one in between standard fuse or breaker sizes. This might not apply directly to air conditioners but I think the code writer's reasoning is the same.

[gfretwell]

Nope You use the label to size the circuit and the breaker. An engineer designed it and an inspector should sign off on it.

The 10 gauge, or even 12 gauge wire is adequately protected from a short circuit by the 40a breaker. Overload protection is provided inside the compressor. That is the way it works on dedicated motor circuits. In fact there is a question on most inspector tests and the right answer is 14 gauge wire, 40a breaker (1 HP motor @ 120v)

[gregz]

I had a 3.5 ton installed. They used 10 gauge and 30 amp breaker. After reading data plate and measuring operating current I switched to 20 amp breaker. It's only drawing 6.5 Amps and no problems. Never hurts to use thicker wire.

Greg

[The Daring Dufas]

The circuit breaker is meant to protect the wiring not necessarily the equipment which will normally have its own secondary protective devices. The 20 amp breaker won't harm anything but if there is a surge current greater than 20 amps from the AC unit starting on a hot day, you may get nuisance tripping of the breaker. O_o

TDD

[trader4]

A 20 amp breaker is less than the rated 24 amp min circuit capacity on the eqpt label. As others have stated, the existing 12 gauge wiring and 40 amp breaker are correct, meet code and no change is required.

[trader4]

Not around these parts. Unless the wiring is too small, deteriorated, etc. If not, then typically just the disconnect box on the exterior is changed.

a new 8 gauge copper line with new disconnect

Why can't you just add those later, when and if needed? It's easier to just connect new wires to new breakers rather than install boxes, deal with splices, etc.

[The Daring Dufas]

Not around here. Every electrical inspector I know would reject a 12 gauge wired circuit with a 40 amp circuit breaker. A 40 amp circuit uses #8 copper NM/UF 60°C cable or #8 aluminum SE/USE 75°C cable. An electrical inspector may require #6 aluminum be used depending on the jurisdiction's requirements which may be stricter than The NEC. This applies to "homes" not necessarily industry which will normally use different wiring methods and higher temperature insulated wiring. I've wired homes, businesses and industry for a living so I know a little about electrical wiring. I've also worked in commercial sales of electrical products. The circuit breaker is meant to protect the wiring and should always be sized accordingly. There are places that have no electrical inspection department and I suppose you can do whatever you want. I've worked in those areas too and seen extremely dangerous wiring that was quite scary. ^_^

TDD

[trader4]

Then they are not following the NEC and either they don't know what they are doing or that particualr AHJ has unusual requirements.

A 40 amp circuit

You're trying to apply the NEC rules for general purpose branch circuits to motor eqpt that is on a dedicated circuit with it's own over-current protection. As the Mike stated, the eqpt label says the minimum circuit ampacity is 24 Amps. That translates into 10 guage wire being fine. Actually, 12 would also meet it. It further states that a 40 amp breaker is the max allowed, which is what he has. Presumably any competent electrical inspector would read the eqpt label and follow it.

Theoretically any AHJ could make up anything they want. But #6 for this 3 ton AC is nuts.

Then you should know that what Mike has is perfectly fine. It exceeds the eqpt label, which is what governs here and it meets NEC.

You're ignoring the fact that the eqpt has it's own over-current protection.

What Mike has is not dangerous. Itfully meets or exceeds NEC in every regard.

[The Daring Dufas]

I'm not going to argue with you over it except to tell you it would not pass inspection in my area. I don't argue with inspectors either because they can call people who carry guns and handcuffs. ^_^

TDD

[trader4]

Then maybe you can tell us what rules the inspectors are following and where your area is, because it's entirely consistent with NEC. And it would pass inspection here, in NJ. Or do your inspectors just make stuff up as they go along?

[The Daring Dufas]

Everywhere I've worked followed the NEC even on U.S. installations overseas where The Army Corps of Engineers took care of inspections. All I know is from my own experience working on various jobs over four decades. Perhaps you have much more experience in the field of electrical work than I do but I can only refer to my own experience. Perhaps you are an experienced electrician in your city/state and know how things are done in your kingdom but from your post, it appears you follow different standards. I must contact my relatives in New Jersey and implore them to run, run like the wind! ^_^

TDD

[TimR]

Except where they conflict with the DIN. Usually you can go with what is most stringent, but sometimes the NEC and DIN conflict and can't both be met.

[trader4]

Then you should be able to cite for us the section of NEC that says the installation that Mikepier has is in violation. Reference please.

I'm an electrical engineer. And gfretw has weighed in on the issue. I believe he's an electrician. And he said the same things I did, in particular that what Mike has is code compliant and does not need to be changed.

We follow the NEC here. So show us where the NEC says we're wrong..... And until them, stop spreading FUD. There is absolutely nothing unsafe, dangerous or in violation of NEC in what Mike has. This is a common confusion. Your mistake is applying the rules for branch circuits for lights, receptacles, etc to HVAC eqpt. Different rules apply for obvious reasons and if you'd just take the time to read the NEC, you'll see that.

[krw]

If you figure a SEER of 12, 3.5T (42,000BTU/HR) is 3.5kW or 14.5A. To get to 6.5A you'd have to have a SEER of 27! Something's not adding up here.

[Ralph Mowery]

Some areas fo the counrty do not exectally follow the NEC. Unless you have a very good reason and can show the inspector where he is mistaken, it is best to just do as they say. If you ever get on their bad side, they will really look over your work and try to find a reason to make you redo something.

It doesn't mater about the gun and cuffs, but they can have the power cut off or not turned on in the first place.